Wind turbines are used to convert mechanical wind energy to electrical energy in a clean and efficient way. In a wind turbine a mechanical drive train comprising a rotor with several rotor blades drives an electric generator, either directly or by means of a gearbox. The resulting alternating current (AC) frequency that is developed at stator terminals of the electric generator is directly proportional to the speed of rotation of the rotor. The voltage at the stator terminals also varies as a function of the rotational speed and the reactive power requirements of the generator. For an optimum energy capture, this rotational speed varies according to the speed of the available wind driving the rotor blades. To limit the energy capture at high wind speeds and to avoid a potential damage of the rotor, the rotational speed of the generator may be controlled by altering the pitch angles of the rotor blades.
An adaptation of the variable voltage and frequency of the electric generator to a nominally fixed voltage and frequency of a power grid is typically achieved by a power converter. A power converter typically includes a generator bridge, which in normal operation operates as an active rectifier to supply power to a direct current (DC) link. The generator bridge can have any suitable topology with a series of semiconductor power switching devices fully controlled and regulated using a pulse width modulation (PWM) strategy. A power converter typically further comprises a network bridge which converts the DC power of the DC link to an AC power output, which in voltage, frequency, and phase angle is matched to the respective electric quantities of the power grid. When transferring or shipping power from the network bridge or from a bus bar being connected to a plurality of network bridges (e.g. via respectively one transformer), apart from the amplitude also the relative phase of the voltage signal at the output of the network bridge or at the bus bar with respect to the phase of the power grid is an important quantity for the amount of power which can be transferred.
In this respect it is mentioned that this phase angle is associated with a certain back Electro Motive Force (back EMF), which in another approach for explaining an electric power transfer is necessary for shipping electric power to the power grid. In this approach the back EMF is generated by the power grid.
By contrast to an AC power connection it is however also possible to transfer the electric power being generated in particular by a plurality of wind turbines being assigned to a wind park to a power or utility grid via a so called High Voltage Direct Current (HVDC) power connection. Such a solution may be in particular appropriate for an offshore wind park or an so called islanded wind park, where the distance between (a) a common bus bar, often also called Point of Common Coupling (PCC), of the wind park, and (b) the respective power receiving (onshore) power grid is large (e.g. hundreds of kilometers). With long distances the electric power losses within an HVDC power transmission system are much smaller than the corresponding losses within an AC power transmission system, in which the inductive power losses caused in particular by the parasitic inductance of the respective cable are much larger.
In the following a power transmission from an offshore wind park via a HVDC power transmission system to an onshore power grid is described:    (1) Each one of a plurality of offshore wind turbines comprises (a) a three phase power converter with a generator (AC-DC) bridge, a DC link, and a three phase network (DC-AC) bridge, and (b) an interface to a medium voltage AC system via a power transformer. Each wind turbine exports AC electrical power from the network bridge into the medium voltage AC power collector system by ensuring that the network bridge modulated voltage has a correct phase angle and magnitude with respect to the medium voltage AC power collector system.    (2) The medium voltage AC power collector system is connected to a high voltage (HV) AC power collector system via a transformer being erected offshore at a substation platform.    (3) The HVAC power output and other HVDC power outputs from other substation platforms are collected at a second bus bar and fed, as a common HVAC power output, to a HVDC platform wherein the common HVAC power output is converted to a DC power output.    (4) The DC power output is transmitted onshore via a (low loss) HVDC cable which may have a length of some more than 100 km.    (5) Onshore the DC power output is fed to a (DC-AC) converter station which generates a modulated AC voltage output. This modulated AC voltage output is controlled with an appropriate voltage and frequency respectively phase angle into the onshore AC power grid so as to export the required power into the onshore AC power grid.
For converting the common HVAC power output into the DC power output at the HVDC platform (see item (3) above), a high power AC-DC converter may be used, which comprises altogether six power semiconductor switches, wherein respectively two power semiconductor switches are connected in series within one (out of three) half bridge paths each extending between the two DC output terminals of the high power AC-DC converter. The power semiconductor switches may be driven in a known manner by means of a Pulse Width Modulation (PWM). Such an AC-DC conversion has the advantage that by providing appropriate switching patterns a bi-directional power flow is possible. However, disadvantages of such an AC-DC conversion are that the high power AC-DC converter is a complex, large and extremely heavy entity. For a reliable operation an air insulation must be provided.
Recently there has been proposed another approach for a AC-DC power conversion at the HVDC platform, which approach is based on the concept of the offshore end of the HVDC system comprising a rectifier having six passive high power diodes. Again, respectively two high power diodes are connected in series within one (out of three) half bridge paths each extending between the two DC output terminals of the corresponding power rectifier. This approach has the advantage that the rectifier can be realized as an encapsulated device and in a simple and robust manner. Power losses within the rectifier are small and the operation of the rectifier does only require comparatively low maintenance costs.
However, a disadvantage of the “rectifier approach” may be that only a one way power flow is possible. In case power has to be transferred from the onshore power grid to the wind park a corresponding HVDC power transmission system must be equipped with a so called umbilical AC cable extending between the onshore power grid and the wind park parallel with respect to the HVDC power cable. A power transfer via the umbilical AC cable may be necessary e.g. during a start-up phase of at least some wind turbines of the wind park when the power generation of the other wind turbines is not sufficient in order to allow for a reliable start-up.
A further challenge when using a (passive) rectifier is that the amplitude, the frequency, and the phase of the offshore HVAC power output which is supposed to be rectified must be controlled exclusively by the DC-AC network bridges of each individual wind turbine.
When operating a wind park several operational modes (OM) may be used, which all required a careful wind turbine control in order to allow for a high operational control. Specifically, in a first operational mode (OM1) the wind park is connected to the utility grid solely via the AC auxiliary power transmission system. In a second operational mode (OM2) the wind park is connected to the AC power grid only via the HVDC power transmission system. In a third operational mode (OM3) the wind park is connected to the AC power grid via both the HVDC power transmission system and umbilical power transmission system.